This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-371883, filed Dec. 27, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a shadow mask, a cathode ray tube comprising the shadow mask, and a method and an apparatus for manufacturing the shadow mask.
In general, a cathode ray tube used in a color television set or the like comprises a vacuum envelope including a panel and a funnel. A phosphor surface including blue (B), green (G), and red (R) phosphor layers and black layers formed therebetween is formed on the inner surface of the panel. In addition, a shadow mask is arranged inside the panel and opposes the phosphor surface.
The shadow mask comprises a mask body, which has a mask surface where a number of electron beam passage apertures are formed and a skirt portion at the peripheral edge part of the mask surface, and a mask frame, which is welded to the skirt portion of the mask body. Holders are welded to the respective corners of the mask frame. Further, panel pins provided on the inner wall of the panel are engaged in installation holes formed in the holder, respectively, thereby supporting the shadow mask at a predetermined position opposing the inner surface of the panel.
The panel thus equipped with the shadow mask is welded integrally to the funnel by frit glass, and thus, a glass valve is constructed as a vacuum envelope. In addition, an electron gun is arranged inside the neck of the funnel, and a deflection yoke is mounted on the outer circumferential surface of the funnel.
The mask body is formed in a predetermined shape by press-molding. The number of passage apertures are regularly arrayed in the mask surface, and the mask surface is formed with a predetermined curvature. In addition, the skirt portion is formed by bending the peripheral edge portion of the mask surface. After the press molding, the mask body is subjected to cleaning and process of blackening, so that a coating of a blackening film made of an oxide film is formed on the surface. This blackening film functions to prevent rust and reflection.
The mask frame of the shadow mask is also subjected to cleaning and process of blackening, after the press molding. The holder is welded to each corner or each side of the mask frame. This mask frame is welded to the outer surface of the skirt portion of the mask body at plural positions. Spot welding is adopted for the welding between the mask body and the mask frame.
According to a welding device used for the spot welding, the mask body is loaded, with the mask surface faced downward, into a lower metal mold which is processed so as to extend along the curved surface of the mask surface. Further, with the mask body kept pressed against the lower metal mold, the mask frame is overlapped on the outer surface of the skirt portion of the mask body, and a pair of electrodes provided at a welding head, which are a pressing-side electrode and a back electrode, are moved in the directions in which they come closer to each other. By this operation, the joining portions of the skirt portion of the shadow mask and the mask frame are clamped with a predetermined pressure between the pair of pressing-side electrode and the back electrode. Further, electric power is conducted between these electrodes so that the skirt portion and the mask frame are subjected to resistance-welding. That is, an electric current flows between the pressing-side electrode and the back electrode. Blackening films respectively formed on the surfaces of the skirt portion and the mask frame are then broken. Thereafter, the skirt portion and the mask frame are welded to each other thereby forming a welding portion called as a nugget.
Normally, iron is used for the mask frame, and iron or invar material is used for the mask body. Since a blackening film which has a low conductivity exists between the pressing-side electrode and the welding material during the welding, splashing occurs when the blackening film is broken or when metals are welded to each other. Splashes may scatter onto the mask surface and may cause clogging of the electron beam passage apertures.
Specifically, a plurality of amphitheatric circular or rectangular openings each having a diameter of 100 to 200 xcexcm are bored in the front and back surfaces of the mask surface of the mask body. Each of these openings has a larger diameter in the side of the surface facing the phosphor surface of the panel and a smaller diameter in the side of the surface facing the electron gun. Each of the electron passage apertures is defined by a pair of larger and smaller diameter openings. Further, the welding between the skirt portion and the mask frame is carried out in a situation that the surface of the mask surface which faces the electron gun is oriented upward, i.e., in a situation that the small diameter openings are faced to the welding portion. Therefore, splashes scattering from the welding portion easily enter into the small diameter openings and causes clogging.
Also, the shadow mask is used in the process of forming the phosphor surface. That is, the phosphor surface is subjected to exposure processing through the apertures of the shadow mask. Specifically, three-color phosphor layers of blue, green, and red are exposed with light which has passed through one of the apertures in the shadow mask. Consequently, a defective phosphor surface is formed if the apertures of the mask body are clogged by splashes, dust, or foreign materials.
Further, at the same time when the blackening film is broken during welding, a larger current flows through the welding portion thereby increasing the temperature of this part. Therefore, the distal end of the pressing-side electrode is worn and comes to be easily oxidized. A problem hence arises in that the pressing-side electrode needs frequent polishing and that the lifetime of this electrode is shortened.
The present invention has been made in view of the above problems and its object is to provide a shadow mask, a cathode ray tube comprising the shadow mask, and a method and apparatus for manufacturing the shadow mask, in which scattering of splashes is reduced so that apertures in the mask body can be prevented from being clogged due to splashes.
To achieve the above object, a shadow mask according to the present invention comprises: a mask body having a mask-effective section where a number of electron beam passage apertures are formed and a skirt portion provided at a peripheral edge of the mask-effective section; and a mask frame arranged outside the skirt portion and resistance-welded to the skirt portion at a plurality of welding portions, wherein the skirt portion includes an outer surface in contact with the mask frame, an inner surface positioned opposite to the outer surface, and a plurality of concave and/or convex portions are formed at the region of the inner surface of the skirt portion with which an electrode for resistance-welding contacts, in each of the welding portions, each of the plurality of concave and/or convex portions having a smaller area than an area of a contact surface of the electrode.
A cathode ray tube according to the present invention comprises: a panel provided with a phosphor screen on an inner surface of the panel; a shadow mask arranged facing the phosphor screen; and an electron gun for emitting an electron beam toward the phosphor screen through the shadow mask, wherein the shadow mask includes a mask body having a mask-effective section where a number of electron beam passage apertures are formed and a skirt portion provided at a peripheral edge of the mask-effective section, and a mask frame arranged outside the skirt portion and resistance-welded to the skirt portion at a plurality of welding portions, and the skirt portion includes an outer surface in contact with the mask frame, an inner surface positioned opposite to the outer surface, and a plurality of concave and/or convex portions are formed at the region of the inner surface of the skirt portion with which an electrode for resistance-welding contacts, in each of the welding portions, each of the plurality of concave and/or convex portions having a smaller area than an area of a contact surface of the electrode.
A method for manufacturing a shadow mask, according to the present invention, comprises the steps of: preparing a mask body having a mask effective section where a number of electron beam passage apertures are formed and a skirt portion provided at a peripheral edge of the mask effective section and having a plurality of concave and/or convex portions formed on an inner surface of the skirt portion; arranging a mask frame layered outside the skirt portion; clamping the skirt portion and the mask frame with a predetermined pressure, at a predetermined welding position, between a first electrode which contacts the inner surface of the skirt portion where the plurality of concave and/or convex portions are formed and a second electrode which contacts the outer surface of the mask frame; and conducting electricity between the first and second electrodes thereby to resistance-weld the skirt portion and the mask frame to each other.
According to the shadow mask, cathode ray tube, and the method of manufacturing a shadow mask which are structured as described above, a plurality of concave and/or convex portions each having a smaller area than the area of the contact surface of a welding electrode are formed on the inner surface of the skirt portion which contacts the welding electrode, at the welding portion between the mask body and the mask frame. Therefore, in welding, the contact area between the skirt portion and the welding electrode is reduced so that the pressure per unit area increases and the current density also increases. As a result, the amount of splashes caused from the welding portion is reduced. In addition, the surface part of the skirt portion is subdivided because a large number of concave or convex portions are formed. Also, the sizes of generated splashes are small because the contact area contacting the electrode is small. Further, it is advantageous that splashes which are smaller than the electron beam passage apertures formed in the mask body clogs no apertures even they scatter to the mask effective section.
Another method of manufacturing a shadow mask, according to the present invention, comprises the steps of: preparing a mask body having a mask effective section where a number of electron beam passage apertures are formed and a skirt portion provided at a peripheral edge of the mask effective section; arranging a mask frame layered outside the skirt portion; clamping the skirt portion and mask frame with a predetermined pressure, at a predetermined welding position, between a first electrode which contacts an inner surface of the skirt portion and a second electrode which contacts an outer surface of the mask frame; surrounding a contact portion between the first electrode and the inner surface of the skirt portion, and a periphery of the first electrode, with a cover for catching splashes; and conducting electricity between the first and second electrodes thereby to resistance-weld the skirt portion and the mask frame to each other.
Further, an apparatus for manufacturing a shadow mask, according to the present invention, comprises: a support portion for supporting a mask body having a mask effective section where a number of electron beam passage apertures are formed and a skirt portion provided at a peripheral edge of the mask effective section, and a mask frame provided to be layered outside the skirt portion; and a welding head for resistance-welding the skirt portion and the mask frame to each other at a predetermined welding position, the welding head including a first electrode which contacts an inner surface of the skirt portion, a second electrode which contacts an outer surface of the mask frame, a press portion for clamping the skirt portion and the mask frame between the first and second electrodes with a predetermined pressure, and a cover for catching splashes, surrounding a contact portion between the first electrode and the inner surface of the skirt portion and a periphery of the first electrode.
According to the method and apparatus described above, the contact portion between the first electrode and the inner surface of the skirt portion, and the periphery of the first electrode are surrounded by the cover. Splashes are caught by this cover. Scattering of splashes can thus be prevented.
Also, according to the present invention, the first electrode is cooled by supplying a cooling medium to the portion surrounded by the cover, so that generation of splashes can be hindered.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.